Drilling systems used to drill wells for oil, gas and other purposes may be thousands of feet underground, change direction, and extend horizontally. To help maximize drilling efficiency, telemetry is used while drilling to transmit data from sensors located downhole to the surface as a well is drilled. Obtaining and transmitting information is commonly referred to as measurement-while-drilling (MWD) and logging-while-drilling (LWD). One transmission technique is electromagnetic telemetry or (EM) telemetry. Typical data includes formation characteristics, well path direction and inclination, and various other drilling parameters. In particular, MWD and LWD systems have used EM tools, located downhole and coupled to sensors along the drill string, to create electric and magnetic fields that propagate through the formation where they may be detected at the surface. EM tools typically include a transmitter, a means, typically called a gap sub, for electrically isolating an upper portion of the drill string from a lower portion of the drill string, an electrode, a power source and sensors. EM tools are configured to convey electrical power from the transmitter mounted in the flow sub, past the insulated portions of the gap sub assembly, to a contact point located uphole from the insulated portion, in order to help create the EM field that conveys drilling data to a receiver on the surface. Because the gap sub assembly is typically located above and attached to the flow sub, a portion of the electrode is typically securely mounted to the inner wall of the gap sub assembly by some means such as bolts or other fasteners, and the transmitter or housing is attached downhole in the flow sub.
Before drilling can begin, multiple components, including the drill bit, special tools, and drill collars, are assembled progressively end-to-end and lowered into the borehole, followed by drill pipe. The various components and drill pipe together form the drill string. Assembling these components and drill pipe is referred to as a “makeup” operation. EM tools are attached to, or positioned, in the drill string during the make-up operation. Fixed mount EM tools, as noted above, are thus mounted inside the drill string during make-up at two spaced apart mount locations: 1) a downhole mount location within the flow sub; and 2) an uphole mount location mount for a contact point above the electrically isolated section of a gap sub assembly. The mount locations typically define electrical connections with the drill string, which are used to help create EM fields used for data transmission.
Ideally, fixed mount EM tools are designed to have the same fixed length between the two mount locations discussed above. In practice at a drill site, this is seldom the case. Drill string components are mixed and reused from drilling one well to another. Variances in distance between the two mount locations are the result of several factors. One factor is manufacturing tolerances in components, e.g. between gap and flow subs. Due to tolerances, gap and flow subs can vary slightly in length, in addition, the specific location of mounting points in these components can vary. Another factor is component wear and application of different amounts of torque from one use to the next. Torque and wear allow the components to screw more or less closely together which result in altering the distance between the two mount locations. Another factor is the need to rework component ends. Worn gap sub and flow sub ends are reworked to ensure a strong connection between adjacent components. Reworking may result in a shorter sub length when the ends are cut off and new threads are machined. Because the mount locations are initially located in separate components, such as the gap sub and flow sub, the drill operator must accommodate the variance in distance between mounting points during a make-up to ensure that electrical connections are formed with the drill string for proper EM telemetry operation later during the drill operation.
Shafts, subs with spacers, bayonet style connectors, and/or telescoping electrode shafts have been used to accommodate variances in distance between the two mount locations or to change the mount locations so that the two mount locations are spaced apart the required distance while still allowing adjacent drill string components to be appropriately connected end-to-end and provide an electrical connection with the drill string. These approaches complicate make-up operation and could comprise EM tool functionality. Use of spacers may require disassembly and reassembly with spacers of a different thickness to achieve alignment. Bayonet connectors accommodate little variance in distance and are subject to infiltration of fluids. Shafts with fixed attachment points are prone to breaking. Telescoping electrode shafts, for instance, are less reliable during drilling due to the penetration of drilling mud solids which may hinder their operation during drilling.